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Notes From the Garden: Repel the Invasion of Japanese Beetles and other Harmful Insects

I get a lot of e-mail questions about gardening since each week this column appears in a dozen papers around New England — and I usually include my email address. Among the most common are ones that read something like this: “Dear Henry, Something is eating my (fill in the blank: roses, cucumbers, broccoli, etc.). What should I do?” Sometimes I will get a photo of a leaf that has been munched, or a blurry bug. Believe me, I’d love to diagnose and give safe remedies. But often I can’t.

Let’s look at bugs, properly called insects. These critters are in the arthropod phylum and all have certain common characteristics: they have an external skeleton, three body sections, jointed legs, compound eyes and a pair of antennae. Other than birds, they are the only creatures that can fly, though many do not. There are over a million named insect species (hence my reluctance to identify them based on a short description). Many insects are beneficial, and many — perhaps most — coevolved with flowering plants. They pollinate our crops and do many wonderful things for us.

When I give lectures I often ask my audience, “What problems have you had with insects?” The most common seems to be with Japanese beetles. These beetles, as the name suggests, are originally from Japan, and were first observed in New Jersey in 1916. In less than 100 years they have become omnipresent in the eastern U.S. Why? They have very few natural predators — even birds don’t want to eat them.

As larvae, these pests generally live in lawns, feeding on grass roots. They are whitish grubs of various sizes, but up to an inch long. If you cut open a square foot of lawn with a sharp shovel and peel back the sod, you are likely to see a grub or two. If you count 10 or more in that sample, you have an infestation that will be a problem.

The U.S. Department of Agriculture developed a remedy in the 1940s called milky spore, a bacterium that can be suspended in water and sprayed on lawns. It is not a miracle cure, and is quite expensive. Not all entomologists believe that milky spore is an effective cure, at least not in New England where weather conditions can kill the bacterium. Not only that, those darn bugs fly. So you can treat your lawn with milky spore only to have your neighbor’s beetles fly over the fence to attack your roses. I talked to an enterprising gardener who convinced her neighbors to treat, too, and she feels it made a significant reduction in beetle numbers.

What else can you do? There are beneficial nematodes (unsegmented worms) called Hb nematodes that will attack Japanese beetle larvae and are said to be 96 percent effective in eliminating Japanese beetle and chafer larvae if applied properly. According to a University of Maine website (just Google “Hb nematode”), the best time to apply these nematodes is July and August when the grubs are feeding in your lawn. If you buy them, follow the directions carefully. They need to be applied to moist lawn at dusk, and then watered in. These are live worms, and as such need to be used soon after purchasing them. They are not generally available at garden centers, but are available online.

What about those Japanese beetle traps? Give them to neighbors you don’t like. They attract lots of beetles, but only capture some — so they attract more hungry beetles to your property if you use them. Really, just don’t buy them.

I am a firm believer that the best method of insect control for most bad bugs is hand-picking them and dropping in soapy water. Insects often have several life cycles in a summer, so try to reduce numbers before they reproduce. Hand picking works for potato bugs, for example, if you check your plants now, before large numbers have appeared. Look under the leaves, too. If you see orange egg masses, scrape them off and drown them in soapy water, along with the beetles and larvae. If you grow too many potatoes for hand picking bugs, try something called “Bt,” another beneficial bacterium. It is readily available at garden centers. It does not act as a contact poison, but sickens the larvae so they stop feeding and don’t reach adulthood.

My insect nemesis is the striped cucumber beetle. It is a small striped beetle that can devour an entire small plant in one night. It eats not only cucumber leaves, but anything in that family including squashes and pumpkins. I do two things to help prevent their destruction: I grow my seedlings in pots until they have three to four leaves so the beetles can’t kill the plant overnight. And I cover my plants with row covers (breathable garden fabric) to keep those darn beetles off the leaves. Which is not to say that they can’t come up under the covers through the soil, but the method does help.

One last thing to consider: if you decide that spraying pesticides is easier than the organic methods described here, know that those same sprays will kill small beneficial insects that you probably never even notice. Those beneficials are keeping most pests from becoming problems.

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Henry Homeyer can be reached at henry.homeyer@comcast.net. His website is www.Gardening-Guy.com.